Responses API: AssistantReasoningDeltaEvent and AssistantReasoningEvent triggered with empty content via BYOK providers (Copilot CLI)

### Describe the bug

When using GitHub Copilot CLI (or SDK) with BYOK—either OpenAI or vLLM—for Responses API, both AssistantReasoningDeltaEvent and AssistantReasoningEvent are triggered but with empty content, even if the provider's response includes reasoning content.

### Affected version

1.0.43

### Steps to reproduce the behavior

1. Set up Copilot CLI to use BYOK, OpenAI or vLLM provider (Windows/WSL). Sample .NET code for SDK (works similarly with CLI):

```csharp
await using var session = await client.CreateSessionAsync(new SessionConfig
{
Provider = new ProviderConfig
{
Type = "openai",
BaseUrl = "https://api.openai.com/v1",
ApiKey = "your_api_key",
WireApi = "responses",
},
ReasoningEffort = "high",
Model = "gpt-5.4-2026-03-05",
OnPermissionRequest = PermissionHandler.ApproveAll,
Streaming = true
});
```

2. Trigger completions or calls that should generate reasoning via the Responses API.
3. See that the events are always empty.

### Expected behavior

These events should present the reasoning/explanation content if the provider response includes it. Content field should not be blank if a reasoning field is present.

### Additional context

- Environment: Windows, WSL
- Copilot CLI version: 1.0.43
- Copilot SDK version: 0.3.0
- BYOK provider: OpenAI or vLLM

実装ファイルやテストは指定されていません。OpenAI または vLLM BYOK プロバイダー向けに示されている Responses API の設定で問題を再現し、その後 reasoning イベントの処理を追跡してください。AssistantReasoningDeltaEvent と AssistantReasoningEvent が空のコンテンツではなくプロバイダーの reasoning コンテンツを公開すれば完了です。

索引モデルが issue の本文から書いたものです。